The present invention relates to a coil driving method, a coil driving apparatus and an MRI (Magnetic Resonance Imaging) apparatus, and more specifically to a coil driving method, a coil driving apparatus and an MRI apparatus capable of preventing variations in sensitivity even if samples inserted between a coil on one side and a coil on the other side, both of which are opposed to each other, are different in body frame.
FIG. 5 is an explanatory view showing examples of a transmitting coil driving circuit and a transmitting coil employed in an MRI apparatus which is related to the present invention.
The transmitting coil driving circuit 9-J is provided with an amplifier 21 for power-amplifying a drive pulse Dp, and a splitter 22 for splitting the output of the amplifier 21 In four for four coils that constitute a transmitting coil 5.
The transmitting coil 5 comprises an upper coil 5A and a lower coil 5B opposite to each other in a vertical direction.
The upper coil 5A comprises an upper first coil 51A ant an upper second coil 52A.
The lower coil 5B comprises lower first coil 51B and a lower second coil 52B.
When power is supplied to the upper first coil 51A, current flows in the form of a figure of 8, so that a magnetic field H1 which extends in the direction orthogonal to a current I1A that travels across the center of the coil and which is parallel with the surface of the coil, is formed in a space away slightly downward from the center of the coil surface.
When power is supplied to the upper second coil 52A, a current flows in the form of a figure of 8, so that a magnetic field H2 which extends in the direction orthogonal to a current I2A that travels across the center of the coil and which is parallel with the surface of the coil, is formed in a space away slightly downward from the center of the coil surface.
The upper first coil 51A and the upper second coil 52A are identical to each other in structure but different 90xc2x0 from each other in the directions of the currents I1A and I2A. Thus, the magnetic fields H1 and H2 intersect at right angles.
While the lower first coil 51B is identical in structure to the upper first coil 51A, the direction of a current I1B, which travels across the center of the coil, is opposite to and parallel with that of the current I1A. Thus, when power is supplied to the lower fist coil 51B, a magnetic field for intensifying the magnetic field H1 is formed.
While the lower second coil 523 is identical En structure to the upper second coil 52A, the direction of a current I2B, which travels across the center of the coil, is opposite to and parallel with that of the current I2A. Thus, when power is supplied to the lower second coil 52B, a magnetic field for intensifying the magnetic field H2 is formed.
In the transmitting coil driving circuit 9-J, power has uniformly been supplied to all the coils 51A, 52A, 51B and 52B from the splitter 22 Such a power supplying method shows no problem because electromagnetic coupling between the upper coil 5A and a sample and electromagnetic coupling between the lower coil 5B and the sample become equivalent to each other in a state in which the sample is inserted into a space defined between the upper coil 5A and the lower coil 5B as shown in FIG. 6.
However, when a sample small in body frame is used as shown in FIG. 7, the sample is brought into a state of being located near the lower coil 5B rather than the upper coil 5A and hence electromagnetic coupling between the upper coil 5A and t he sample and electromagnetic coupling between the lower coil 5B and the sample are brought into unbalance. Therefore, the uniform supply of the power from the splitter 22 to all the coils 51A, 52A, 51B and 52B yields the unbalance between a magnetic field formed by the upper coil 5A and a magnetic field formed by the lower coil 5B, thus causing a problem that variations in sensitivity occur.
Therefore, an object of the present invention is to provide a coil driving method, a coil driving apparatus and an MRI apparatus capable of preventing variations in sensitivity even if samples inserted between a coil on one side and a coil on the other side both of which are opposed to each other, are different in body frame.
In a first aspect, the present invention provides a coil driving method comprising steps of inserting a sample between a coil on one side and a coil or the other side both of which are disposed so as to be opposite to each other, adjusting a balance in the supply of power to the both coils and thereafter supplying power to the both coils.
In the coil driving method according to the first aspect, the balance in the supply of the power to both coils is adjusted after the sample is actually inserted into a space defined between the coil on one side and the coil on the other side. Therefore, a magnetic filed formed by the coil on one side and a magnetic field formed by the coil on the other side always become equal to each other regardless of a body frame and a position of the sample, thus causing no variations in sensitivity.
In a second aspect, the present invention provides a coil driving method having the above configuration, which includes steps of measuring reflected waves sent from the both coils and controlling a power-supply balance so that the voltages of the reflected waves coincide with each other.
The coil driving method according o the second aspect has an advantage in that since the power-supply balance is adjusted with the voltages of the reflected waves from the coil on one side and the coil on the other side as indexes, the coil on one side and the coil on the other side need no work.
In a third aspect, the present invention provides a coil driving method having the above configuration, wherein magnetic field detecting means are provided in the vicinity of the coils and further including a step of controlling a power-supply balance so that the magnitudes of signals detected by the magnetic field detecting means coincide with each other.
The coil driving method according to the third aspect has an advantage in that since magnetic fields generated by the coil on one side and the coil on the other side are actually measured to adjust the power-supply balance, the power-supply balance can directly be adjusted as compared with the case where the voltages of the reflected waves are set as the indexes.
In a fourth aspect, the present invention provides a coil driving method comprising steps of controlling a balance in the supply of power to a coil on one side and a coil on the other side, both of which are disposed so as to be opposite to each other, the balance being determined in advance in association with a body frame of a sample inserted between the two coils, and thereafter supplying power to the both coils.
A shift in the balance of the supply of power to each of the coil on one side and the coil on the other side depends on the body frame of the sample. In other words, the amount of compensation for the shift in the power-supply balance can be recognized from the body frame of the sample.
In the coil driving method according to the fourth aspect, the power-supply balance is adjusted according to the amount of compensation determined in advance in association with the body frame of the sample. Therefore, a magnetic filed formed by the coil on one side and a magnetic field formed by the coil on the other side can evenly be adjusted, thus causing no variations in sensitivity. Since it is also unnecessary to actually insert the sample into a space defined between the coil on one side and the coil on the other coil and measure the power-supply balance, an advantage is brought about in that a work load is lightened.
In a fifth aspect, the present invention provides a coil driving method having the above configuration, including a step of using the weight of the sample as the body frame of the sample.
While the weight of the sample and its body thickness may be used as the body frame of the sample which causes a shift in the balance in the supply of power to the coil on one side and the coil on the other side, the weight is apt to be best handled.
Since the weight is used as the body frame of the sample in the coil driving method according to the fifth aspect, its handling becomes the simplest.
In a sixth aspect, the present invention provides a coil driving method having the above configuration, wherein the coil on one side comprises a first coil on one side and a second coil on one side, magnetic fields formed by the first coil on one side and the second coil on one side intersect at right angles in a coil-to-coil space defined between the coil on one side and the coil on the other side, the coil on the other side comprises a first coil on the other side and a second coil on the other side, a magnetic field formed by the first coil on the other side is parallel with a magnetic field formed by the first coil on one side within the coil-to-coil space, and a magnetic field formed by the second coil on the other side is parallel with a magnetic field formed by the second coil on one side within the coil-to-coil space.
In the coil driving method according to the sixth aspect, the present invention can be applied to a transmitting coil employed in an MRI apparatus.
In a seventh aspect, the present invention provides a coil driving method having the above configuration, wherein the coil on one side and the coil on the other side are opposed to each other in a vertical or horizontal direction.
In the coil driving method according to the seventh aspect, the present invention can be applied to a transmitting coil employed in an MRI apparatus.
In an eighth aspect, the present invention provides a coil driving apparatus comprising power supplying means for supplying power to a coil on one side and a coil on the other side, both of which are placed so as to be opposed to each other, power-supply balance measuring means for measuring a balance in the supply of power to the both coils, and power-supply balance control means for adjusting the balance in the supply of power to the both coils.
The coil driving method according to the first aspect can suitably be executed in the coil driving apparatus according to the eighth aspect.
In a ninth aspect, the present invention provides a coil driving apparatus having the above configuration, wherein the power-supply balance measuring means includes reflected wave measuring means for measuring reflected waves sent from the coils.
The coil driving method according to the second aspect can suitably be carried out in the coil driving apparatus according to the ninth aspect.
In a tenth aspect, the present invention provides a coil driving apparatus having the above configuration, wherein the reflected wave measuring means is a directional coupler.
In the coil driving apparatus according to the tenth aspect, the coil driving method according to the second aspect can suitably be implemented using the directional coupler.
In an eleventh aspect, the present invention provides a coil driving apparatus having the above configuration, wherein the power-supply balance measuring means includes magnetic field detecting means provided in the vicinity of the coils.
In the coil driving apparatus according to the eleventh aspect, the coil driving method according to the third aspect can suitably be carried out.
In a twelfth aspect, the present invention provides a coil driving apparatus having the above configuration, wherein the magnetic field detecting means are pick-up coils.
In the coil driving apparatus according to the twelfth aspect, the coil driving method according to the third aspect can suitably be implemented using the pick-up coils.
In a thirteenth aspect, the present invention provides a coil driving apparatus comprising power supplying means for supplying power to a coil on one side and a coil on the other side, both of which are placed so as to be opposed to each other, and power-supply balance control means for adjusting a balance in the supply of power to the both coils, which is determined in advance in association with a body frame of a sample inserted between the both coils.
In the coil driving apparatus according to the thirteenth aspect, the coil driving method according to the fourth aspect can suitably be implemented.
In a fourteenth aspect, the present invention provides a coil driving apparatus wherein in the coil driving method configured as described above, the weight of the sample is used as the body frame of the sample.
In the coil driving apparatus according to the fourteenth aspect, the coil driving method according to the fifth aspect can suitably be implemented.
In a fifteenth aspect, the present invention provides a coil driving apparatus having the above configuration, wherein the coil on one side comprises a first coil on one side and a second coil on one side, magnetic fields formed by the first coil on one side and the second coil on one side intersect at right angles within a coil-to-coil space defined between the coil on one side and the coil on the other side, the coil on the other side comprises a first coil on the other side and a second coil on the other side, a magnetic field formed by the first coil on he other side is parallel with a magnetic field formed by the first coil on one side within the coil-to-coil space, and a magnetic field formed by he second coil on the other side is parallel with a magnetic field formed by the second coil on one side within the coil-to-coil space.
In the coil driving apparatus according to the fifteenth aspect, the coil driving method according to the sixth aspect can suitably be implemented.
In a sixteenth aspect, the present invention provides a coil driving apparatus having the above configuration, wherein the coil on one side and the coil on the other side are opposed to each other in a vertical or horizontal direction.
In the coil driving apparatus according to the sixteenth aspect, the coil driving method according to the seventh aspect can suitably be implemented.
In a seventeenth aspect, the present invention provides an MRI apparatus equipped with the coil driving apparatus configured as described above.
In the coil driving apparatus according to the seventeenth aspect, the coil driving method according to the seventh aspect can suitably be implemented.
According to a coil driving method, a coil driving apparatus and an MRI apparatus of the present Invention, even if samples inserted between a coil on one side and a coil on the other side, both of which are opposite to each other, are different in body frame, a magnetic field formed by the coil on one side and a magnetic field formed by the coil on the other side are always brought into uniformity, so that variations in sensitivity is prevented from occurring.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.